The present invention relates to the field of optical position/distance measurement systems. More particularly, the present invention is an optical position sensor using coherent (heterodyne) detection and polarization preserving (birefringent) optical fiber. The coherent detection enables high precision accuracy to be obtained, while the polarization preserving fiber enables the system to be utilized in areas of limited access.
Many optical systems exist which measure a distance to a target. Such systems utilize an open beam propagated through free space between the laser source and the target. However, when the target location is such that limited free space is available for beam propagation, such known systems are of limited use. Thus, known systems may be able to perform distance measurements, but the open beam optical sensor head prevents application in limited access areas and tight places. For example, precision measurement of dimensions inside a chassis cannot be accomplished with known open beam systems. While it is known to transfer light through optical cables, precision is compromised due to the environmental effects on the fiber itself. These environmental effects change the optical path length and the polarization of the light in the fiber, adversely affecting measurement precision.
A known optical measurement system is disclosed in U.S. Pat. No. 4,340,304 to Massie. Massie discloses an interferometric method and system for detecting defects in the surface of a mirror. Massie discloses a polarizing beam splitter, a quarter-wave plate, and a target (test mirror). However, Massie is an open beam system and thus incapable of accessing limited space targets. Furthermore, Massie utilizes two laser beams orthogonally polarized with respect to each other. One of the beams is frequency or phase modulated. The beams are heterodyned together, but may not provide the precision capable in coherent systems utilizing a single source light beam.
U.S. Pat. No. 3,771,875 to Russo also teaches an optical interferometer. In Russo, the interferometer has a DC level compensation device. The laser provides a source light beam which is split into a target beam and a reference beam. The target and reference beams also have different polarizations. Thus, the precision available in the Russo device also suffers since the two beams are not in the same spatial mode. Furthermore, Russo is also an open beam system, and thus inapplicable to limited access measurements.
U.S. Pat. No. 4,563,091 to Danliker discloses an open beam system for measuring a position and orientation of a target. However, Danliker fails to suggest coherent optical detection or the use of polarization preserving fiber to gain access to tight places.
Recently, advances in optical technology have enabled the use of coherent (heterodyne) optical detection techniques. Such coherent techniques can provide a 1000 fold increase in the amount of information which can be derived from the light beam reflected from the target. The techniques and advantages of coherent optical detection are generally described in the co-pending U.S. application Ser. No. 590,350 entitled "FREQUENCY MODULATED LASER RADAR", the teachings of which are incorporated herein by reference. Additionally, the article entitled "COHERENT OPTICAL DETECTION; A THOUSAND CALLS ON ONE CIRCUIT" by Link and Henry, IEEE SPECTRUM, February 1987, pp. 52-57 describes the present state of optical heterodyne reception. The teachings of this article are also incorporated into this application by reference.
The advantages of coherent optical detection are fundamental. The information carrying capacity of the optical beam reflected from the target is orders of magnitude greater than other available systems. Simply put, the use of optical heterodyne detection allows for optical radiation detection at the quantum noise level. As such, coherent optical systems provide greater range, accuracy, and reliability than many known prior art measurement systems. This means that rough surfaces and diffuse targets may now be measured. Coherent optical systems can also provide a greater scanning range, a greater working depth of field, and may also operate in ambient light conditions. Furthermore, in a coherent system the target beam is not required to dwell upon the target for very long in order to obtain sufficient information about the characteristics of that target location.
Briefly, optical heterodyne detection provides a source light beam which is directed to a target and reflected therefrom. The return light beam is then mixed with a local oscillator light beam on a photo detector to provide optical interference patterns which may be processed to provide detailed information about the target. Optical heterodyne techniques take advantage of the source and reflected light beam reciprocity. For example, these light beams are substantially the same wavelength and are directed over the same optical axis. This provides an improved signal-to-noise ratio (SNR) and heightened sensitivity. The SNR is sufficiently high so that a small receiving aperture may be used, in contrast to known direct detection systems. A small receiver aperture may be envisioned as a very small lens capable of being inserted into limited access areas. Since a small receiver aperture can still provide detailed information about the target, the optical components of a coherent system may be made very small and provide related increases in scanning speed and accuracy. For example, a coherent optical system using a one half inch aperture can obtain much more information about a target than a four inch aperture used in a direct optical detection system.
The prior art shows that several laser systems have been applied to metrology, and to some extent to gauging. The best known of these is the interferometer which has become a standard for measurement systems. However, the interferometer only measures changes in distance and must be implemented with precisely oriented cooperative reflecting targets. In contrast, the present invention achieves precise measurement of absolute distances of ordinary and rough surfaces. Other prior art laser applications to gauging achieved distance measurements with incoherent detection and triangulation of a laser source and detection system. The accuracy and versatility of such systems are extremely limited.
Key technologies of Al Ga As laser diodes and fiber optical components are currently enjoying a burst of development for applications in telecommunications. Because of these efforts, recent improvements in the quality of injection laser diodes provide the coherence length and wavelength tuning range needed for a precision, coherent optical measurement system. The small size of the injection laser diode and high-technology integrated optical assemblies make possible the development of a new family of small, low cost, precise distance measuring devices which are orders of magnitudes more accurate and more reliable than their more conventional counterparts.
The fundamental concept of coherent optical detection used in the present invention is based on the continuous wave (CW) radar principle. The optical source produces a continuous beam of radiation which is directed at the target. A local oscillator beam is derived from the source light beam and directed to a photo detector. Light reflected from the target is also directed to the photo detector. Since the detector sees energy reflected from the target as well as directly from the source, interference beats are detected as the frequency of the source beam is swept over the interval .DELTA.f. The rate of these beats is a function of the range as well as the magnitude of the frequency interval. This technique allows a tremendous amount of information concerning the target to be derived from the reflected light beam.
One coherent optical detection system is described in U.S. Pat. No. 4,611,912 to Falk et al. Falk et al '912 describes a method and apparatus for optically measuring a distance to and the velocity of a target. In Falk et al, a laser diode provides a linearly polarized, amplitude modulated (with frequency modulated subcarrier) source light beam. The source light beam is directed to a polarization dependent beam splitter which reflects it toward the target. Between the beam splitter and the target is disposed a quarter-wave retardation plate which converts the linearly polarized source light beam to right-hand circularly polarized optical radiation. Between the quarter-wave plate and the target, a local oscillator reflector plate reflects approximately 1% of the source light beam back toward the beam splitter, while allowing approximately 99% of the source light beam to pass toward the target. Light reflected from the target, and light reflected from the local oscillator reflector plate are thereby converted from right-hand circularly polarized optical radiation to left-hand circularly polarized optical radiation. These beams then pass back through the quarter-wave plate and are thereby converted to linearly polarized light beams. These linearly polarized light beams pass through the polarizing beam splitter and are concentrated on a PIN diode by a collecting optical lens. Thus, the local oscillator and the return beam are both linearly polarized in the same direction and are directed along the same optical axis. Thus, the PIN diode detects an optically mixed signal containing the local oscillator beam and the light beam reflected from the target.
However, an extreme disadvantage of the Falk et al '912 system is that very close optical alignment is required between the components. Thus, the laser diode, the beam splitter, the quarter-wave plate, the PIN diode, and especially the local oscillator reflecting plate must be carefully adjusted before usable signals may be obtained. Such close adjustment allows for rapid system degradation with temperature changes and mechanical shocks. Additionally, the Falk et al '912 system is an open beam system and thus incapable of accessing limited areas.
Thus, what is needed is a practical optical precision measurement system capable of great accuracy, rapid measurement time, access to tight spaces, flexibility, and reliability. The present invention proposes such a system.